1. Field of the Invention
The present invention relates to a method of forming a thin film onto a semiconductor substrate using a plasma CVD apparatus, particularly to a method of preventing the semiconductor substrate from being adsorbed on a susceptor.
2. Description of the Related Art
A plasma CVD apparatus has been heretofore generally used in order to form or remove a thin film or to modify the surface of a material to be treated. A basic technique of forming the thin film on a semiconductor substrate (e.g., silicon) or a glass substrate or etching the thin film is now indispensable especially for manufacturing memory chips or devices, CPUs or other semiconductor devices, or liquid crystal displays (LCD).
A plasma CVD process includes steps of introducing various material gasses into an evacuated reaction container, applying a radio-frequency power to the container to generate a plasma therein, and forming various thin films on the semiconductor substrate.
The plasma CVD apparatus generally constitutes a reaction chamber, an upper radio-frequency electrode (showerhead) which also serves as a gas dispersion plate for uniformly distributing the reaction gas, and a lower radio-frequency electrode which also serves as a susceptor for holding the semiconductor. The reaction chamber is connected to a transfer chamber via a gate valve. A conveying robot for conveying the semiconductor substrate into or out of the reaction chamber is installed inside the, transfer chamber. At least three lift pins are vertically passed through the susceptor. As the susceptor moves downward, these lift pins push up the semiconductor substrate placed on the susceptor.
In general, the conveying robot conveys a semiconductor substrate onto a susceptor from which the lift pins project, and slowly places the semiconductor substrate on the lift pins. Thereafter, when the susceptor moves upward and the lift pins move downward with respect to the susceptor, the semiconductor substrate is held on the surface of the susceptor, thereby starting a film-forming treatment. When the film-forming treatment ends, the susceptor moves downward, and the lift pins projecting from the susceptor detach the semiconductor substrate from the susceptor to hold the semiconductor substrate in mid-air. Finally, the conveying robot conveys the treated semiconductor substrate to the transfer chamber from the reaction chamber.
Usually, the semiconductor substrate subjected to plasma treatment is electrostatically charged with plasma and electrostatically attracted to the surface of the susceptor. If the electrostatically adsorbed semiconductor substrate is forcibly detached from the susceptor by the lift pins, the position of the semiconductor substrate shifts due to impact during detachment, and the semiconductor substrate cannot automatically be conveyed by the conveying robot. Moreover, if the impact is too strong, the semiconductor substrate is sometimes broken.
A method of preventing an error from being caused during conveyance or preventing a semiconductor substrate from breaking is disclosed, for example, in Japanese Patent No. 2890494 and U.S. Pat. No. 5,380,566. The disclosed method includes steps of interrupting a material gas, which contributes to film formation, immediately after completing film-forming treatment using plasma, thereby stopping film growth, and gradually lowering radio-frequency power to reduce the charge of the semiconductor substrate. Another method of preventing conveyance errors or semiconductor substrate breakage is disclosed in Japanese Patent Application Laid-Open No. 340896/1998 in which the susceptor surface is provided with irregularities and the contact area between the semiconductor substrate and the susceptor surface is reduced to prevent the semiconductor substrate from being attracted to the surface of the susceptor.
On the other hand, with enhancement of the density of a semiconductor device, there has been a necessity of preventing metal contamination from being caused by a metallic susceptor or a heater. To solve this problem, a ceramic heater has been proposed. The heater is manufactured of alumina ceramic (Al2O3) or aluminum nitride (AlN) which has a resistance to the plasma, so that impurity contamination is reduced. The ceramic heater also serves as the susceptor for directly holding the semiconductor substrate, and a resistance heating wire and a radio-frequency electrode are embedded in the heater. The radio-frequency electrode is embedded at a depth of several hundreds to several thousands of micrometers from the surface of the heater which directly contacts the semiconductor substrate.
However, when such a heater is used, the semiconductor substrate is strongly electrostatically attracted to the susceptor, and the conveyance error frequently occurs. In this case, in the method of gradually lowering the radio-frequency power as disclosed in the Japanese Pat. No. 2890494 and U.S. Pat. No. 5,380,566, the charge of the semiconductor substrate cannot sufficiently be decreased. Moreover, it has also been found out that even in the method of providing the irregularities on the susceptor surface as disclosed in the Japanese Patent Application Laid-Open No. 340896/1998, the adsorption of the semiconductor substrate on the susceptor cannot sufficiently be prevented.
Furthermore, the method of gradually lowering radio-frequency power as disclosed in the Japanese Patent No. 2890494 and U.S. Pat. No. 5380566 has a disadvantage that productivity decreases with a lapse of time which does not contribute to the film formation.
Therefore, an object of the present invention is to provide a method of forming a thin film on a semiconductor substrate, so that a semiconductor substrate is not attracted to a susceptor surface and no conveyance error occurs.
Another object of the present invention is to provide a method of forming a thin film on a semiconductor substrate with little impurity contamination.
A further object of the present invention is to provide a method of forming with high productivity a thin film on a semiconductor substrate.
To achieve the aforementioned objects, the present invention includes a method of using a plasma CVD apparatus comprising a reaction chamber and a susceptor to form a thin film on a semiconductor substrate, wherein the method comprises a pretreatment step of forming a surface layer on the susceptor, so that electrostatic adsorption of the semiconductor substrate on the susceptor is prevented by the surface layer.
The pretreatment step preferably comprises the steps of: introducing into the reaction chamber a gas containing the same gas as that for use in a film-forming treatment in which the thin film is formed on the semiconductor substrate into the reaction chamber; and forming the surface layer on the surface of the susceptor by a CVD process.
The pretreatment step may preferably be executed in the reaction chamber immediately before subjecting at least one semiconductor substrate to a film-forming treatment.
Moreover, the pretreatment step may preferably be executed every time a cleaning sequence in the reaction chamber ends.
The surface layer may be formed of a material lower in resistivity than the susceptor surface, specifically a material whose electric resistivity is 10xe2x88x925 times or less that of the susceptor surface.
Specifically, the surface layer may be formed of a material selected from the group consisting of amorphous silicon, polysilicon, silicon carbide, tungsten, tungsten nitride, tantalum, and tantalum nitride.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.